Solid state semiconductor lasers are important devices in optoelectronic communication systems and high-speed printing systems. Although edge emitting lasers are currently used in the vast majority of such applications, recently, there has been an increased interest in vertical cavity surface emitting lasers ("VCSELs") therefor. One reason for the interest in VCSELs is that edge emitting lasers produce a beam with a large angular divergence, making efficient collection of the emitted beam more difficult. Furthermore, edge emitting lasers cannot be tested until the wafer is cleaved into individual devices, the edges of which form the mirror facets of each device. On the other hand, the beam of a VCSEL has a small and circular angular divergence, and the VCSEL is a stacked structure that incorporates the mirrors monolithically into its layer design. Therefore, it allows for on-wafer testing and for monolithic fabrication of one-dimensional or two-dimensional laser arrays.
A known technique to fabricate a VCSEL's laser emitting aperture is by a lateral oxidation process. In this process, the stack of epitaxial layers comprising the laser device includes an AlGaAs layer with a high aluminum content disposed either above or below the active layer. Then, a patterned etching is performed, forming a mesa structure and exposing the edges of the stacked layers. To form the lasing aperture or the emissive region, the structure is exposed to an oxidizing environment which will cause only the high aluminum content AlGaAs layer to be oxidized laterally inwardly from its free edges towards the center of the mesa structure. Other layers in the structure remain essentially unoxidized since their aluminum content is lower. Thus, the oxidized portions of the high aluminum content layer become electrically non-conductive and the remaining unoxidized portions, which remain conductive, form the laser aperture, which directs the current path through the VCSEL structure. A VCSEL formed by such a technique is discussed in "Selectively Oxidized Vertical Cavity Surface Emitting Lasers With 50% Power Conversion Efficiency," Electronics Letters, vol. 31, pp.208-209 (1995).
One disadvantage of this approach is that the oxidation process results in poor control over the shape of the aperture. The process produces apertures with uneven and jagged boundaries because the rate of oxidation through different portions of the high aluminum content AlGaAs layer depends upon the direction of the oxidation fronts relative to the crystal orientation.
Another disadvantage of this approach is the difficulty in controlling the amount of oxidation in order to accurately define the aperture. A typical mesa for a VCSEL structure is on the order of 50 to 100 microns (.mu.m) across and the desired device aperture is generally on the order of one to tens of microns. Therefore, several tens of microns of lateral oxidation would typically be required in order to fabricate the device. Since the size of the resulting aperture is small relative to the extent of the lateral oxidation regions, the devices formed will have severe variations in their performance characteristics, aperture sizes, and aperture shapes. This lateral oxidation approach is also sensitive to composition non-uniformity and fluctuation in processing temperatures. These shortcomings decrease the reproducibility of device performance and result in, manufacturability and yield problems.
Accordingly, there is a need for developing VCSEL structures with well-defined and well-controlled apertures. These structures should also allow for the formation of high density and high uniformity laser arrays.